1. Field of the Invention
Broadly, this invention relates to a polymer pelletizing apparatus and to a system for controlling the position of pelletizing (cutting) knives or blades relative to a die in the polymer pelletizing apparatus. More particularly, this invention relates to a method and apparatus for electrically and automatically controlling the advancement of pelletizing knives relative to the die in the polymer pelletizing apparatus. In a specific embodiment, the invention relates to a system that advances (indexes) pelletizing knives a preset (predetermined) distance upon the expiration of a preset (predetermined) time period.
2. Description of Related Art
A polymer pelletizing apparatus typically includes a die for directing a molten polymer toward a series of pelletizing knives that cut the polymer into pellets. The pelletizing knives are attached to a drive shaft that rotates and can be advanced closer toward the die. The knives are designed so that when they are in proper contact with the die, they cut appropriately shaped and sized pellets. The contact between the rotating knives and the die causes the knives to be sharpened. Consequently, the knives also wear down. As the knives wear down, the distance between the die and the knives becomes larger. Although the distances involved are microscopic, they are nevertheless sufficient in magnitude to gradually cause the dimension and shape of the pellets cut by the knives to become irregular.
The present inventors have recognized that manually adjusting the knives creates problems because the knives are often moved too close to the die surface, resulting in excessive wear on the knives, and sometimes causing the knives to even break. On the other hand, the inventors have also recognized that with manual adjustment the knives may not be moved close enough to the die, i.e., resulting in excessive distance. The inventors have observed that this excessive distance causes the knives to become dulled as well as causing the production of irregular pellets. Still other times, the knives were not advanced at a proper time in that the manual adjustment was not made frequently enough, producing irregular polymer pellets, or was made too frequently, causing the knives to prematurely wear or break.
One approach is to maintain the knives in substantially constant pressure with the die. The inventors have observed that this approach does not adequately solve the problem because constant pressure causes the knives to wear too quickly, requiring a greater frequency of knife changes. Also, the necessary pressure is imprecisely determined and applied, often causing the knives to break. Furthermore, with these constant pressure devices, there is no way to predict when a knife change is needed. Thus, oftentimes, the pelletizing apparatus must be shut down to correct or adjust mechanical features or problems, and then later have to be shut down again for a knife change. In polymer pelletizing operations, these shutdowns cause great expense and inconvenience. If the need for a knife change could be accurately predicted, the pelletizing system could be shut down one time, to correct or adjust mechanical features and to change the knives. Accordingly, a need exists for not only controlling the indexing of the pelletizing blades to maximize their useful life, but also to accurately predict the exact time when such useful life is reached. In certain aspects, the present invention is directed to overcoming one or more shortcomings of previous methods and meeting needs of maximizing blade life and predicting blade changes.